Bayer Aktiengesellschaft

Claus Rathjen

Artur Reischl

Martin Ullrich

Bernd Quiring

Ernst-Willi Muller

The organic substance present in the form of a melt is cooled below its solidification point in a twin-screw extruder so that it crystallizes out. The crystallized material is then extruded through a breaker plate at the end of the screw extruder and further cooled. It is essential that the temperature prevailing in the screw extruder, its rotational speed and the temperature of the breaker plate should be adjusted such that from 70% to 99.5% and preferably from 95 to 99.5% of the material has solidified by the time it reaches the end of the screw, the non-crystallized liquid residue only crystallizing out during the subsequent cooling operation. Accordingly, a small residue of melt is present during extrusion through the breaker plate. In addition, extrusion is facilitated if the breaker plate is kept at a temperature in the vicinity of the melting point of the material in question.

Process for the production of granulates of organic substances present

The product which is present in the form of a melt is cooled and thereby crystallized in a double-shaft worm machine which has self-cleaning worm shafts rotating in the same direction. The crystallizing material is continually crushed in a repeatedly cutting manner during solidification and during the subsequent cooling operation, while maintaining the main transporting direction in the worm. In this manner, crystal agglomerates are prevented from forming which have only solidified on the surface, while still containing liquid portions inside. The process is carried out using a worm machine, the worm profile of which is repeatedly interrupted by introducing grooves, so that only small segment ridges remain.

Process and an apparatus for the crystallization of melts with a

The instant invention is directed to a process for the continuous preparation of polyisocyanates containing uretdione groups comprising dimerizing aromatic polyisocyanates which are free from uretdione groups in the presence of catalysts which accelerate the dimerization of isocyanates, at -30.degree. to 90.degree. C. in a self-cleaning, coolable screw reactor with continuous feeding in of the free polyisocyanate and of the dimerization catalyst, and in the presence of from 3 to 90% by weight, based on the whole mixture, of an additive containing hydroxyl groups, which additive comprises either (i) a compound within the molecular weight range of from 62 to 10,000 containing at least one hydroxyl group or (ii) a mixture of several compounds having an average molecular weight within the range of from 62 to 10,000 containing at least one bound hydroxyl group, wherein said hydroxyl-containing compound or compounds are added to the reaction mixture before the isocyanate used as starting material solidifies due to the dimerization reaction.

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Leverkusen, Germany

Claus Rathjen